The chart of accounts is the list of accounts your bookkeeping posts to — revenue, cost of goods, VAT, bank, debt and so on. It is the foundation: almost everything else in the accounts builds on top of it.
Reports and overview are only as good as the chart of accounts. A messy chart gives messy figures.
VAT and bookkeeping depend on postings hitting the correct accounts.
Changing it later is possible, but the more you have posted, the more troublesome the cleanup becomes.
Which accounts you need for your particular business.
How income and costs should be grouped, so the reports make sense to you.
The chart of accounts is closely tied to your accounts and your accountant's needs. We recommend establishing it together with your consultant or accountant before you start posting in earnest.
Read more in Chart of accounts.